If
folic acid supplements are given to treat folate deficiency, which can be caused by the use of
phenytoin, the
serum phenytoin levels may fall, leading to decreased seizure
control in some patients. Reductions in
serum phenytoin levels of 16 to 50% have been described with
folic acid doses between 5 mg and 15 mg; doses as low as
folic acid 1 mg may interact.
Monitor
phenytoin levels and adjust the dose accordingly. Fosphenytoin is a prodrug of
phenytoin. Although it does not appear to have been studied, it may interact similarly.